In known roof windows of this kind the fastening device is provided as one or more screws that are screwed through the covering and into the underlying window structure, i.e. either the frame or the sash.
This has worked very well for decades, but in view of the ever increasing demand on the windows, such as ease of mounting, longer lifetime, easier maintenance and improved insulating properties, it is the object of the invention to provide a window with an alternative fastening of the covering.